This invention concerns a method for enlarging a bore hole, either straight or arcuate.
A prior art device is disclosed in U.S. Pat. No. 4,674,479 for enlarging an existing pilot bore hole in the ground, in which the pointed end of a conical enlarging head is fitted to a tubular member that has been installed in a pilot bore hole and is drawn by the same through the pilot bore hole During this process, fluid is supplied to the enlarging head, forced under high pressure through the tubing. The enlarging head has a plurality of passages which extend longitudinally, substantially in the direction of movement, as well as transversely thereto, and which are in fluid communication with the inner bore of the tubular member. Injection nozzles located at the outer ends of the passages are provided for producing a fine liquid high-pressure jet which is used to cut through the ground.
Due to the pull exerted on the tubing and the rotating motion of the tubing, the cone-shaped enlarging head is steadily pulled while being forced, rotatingly into the earth mass split by the fluid jet, whereby the pilot hole is enlarged.
Thus, the hole enlarging procedure is comprised of a rotational movement and a steady axial thrust of the enlarging head into the earth mass surrounding the pilot hole. During this hole enlarging procedure the associated tubing is also being pulled through the bore hole.
The disadvantage of this prior art procedure is that difficulties will arise when working in ground that is relatively solid, or in ground with rock inclusions. While it is still possible to drill the pilot hole in this type of soil, because the pilot hole drilling device is able to laterally bypass the rock inclusions, difficulties are encountered during the hole enlargement process because the rock inclusions must be either displaced in the solid soil, or parts of the rock inclusions must be separated. This cannot be accomplished with the prior art device described above.
In densely compacted ground, even ground without rock inclusions, the enlarging procedure of the pilot hole in the ground often times cannot be carried out only by flushing or cutting and with the application of static traction forces, because the enlarging head diameter is approximately 3 to 4 times the size of the pilot hole diameter. As a result, the cross-sectional area of the pilot bore is enlarged by 9 to 16 times to the cross section of the enlarged bore, and the peripheral wall is being enlarged accordingly. When applying purely static traction forces and pressures there is also the risk that the large frictional forces acting on the peripheral wall of the tubing can no longer be overcome because of the surrounding soil masses.
Therefore, it is the object of the present invention to provide a method and apparatus for enlarging bore holes in solid soil masses or soils with rock inclusions which eliminates the disadvantages described above.